A new approach to estimation of global air‐sea gas transfer velocity fields using dual‐frequency altimeter backscatter

Abstract

A new approach to estimating air‐sea gas transfer velocities based on normalized backscatter from the dual‐frequency TOPEX and Jason‐1 altimeters is described. The differential scattering of Ku‐band (13.6 GHz) and C‐band (5.3 GHz) microwave pulses is used to isolate the contribution of small‐scale waves to mean square slope and gas transfer. Mean square slope is derived for the nominal wave number range 40–100 rad m−1 by differencing mean square slope estimates computed from the normalized backscatter in each band, using a simple geometric optics model. Model parameters for calculating the differenced mean square slope over this wave number range are optimized using in situ optical slope measurements. An empirical relation between gas transfer velocity and mean square slope, also based on field measurements, is then used to derive gas transfer velocities. Initial results demonstrate that the calculated transfer velocities exhibit magnitudes and a dynamic range which are generally consistent with existing field measurements. The new algorithm is used to construct monthly global maps of gas transfer velocity and to illustrate seasonal transfer velocity variations over a 1‐year period. The measurement precision estimated from >106 duplicate observations of the sea surface by TOPEX and Jason‐1 altimeters orbiting in tandem is better than 10%. The estimated overall uncertainty of the method is ±30%. The long‐term global, area‐weighted, Schmidt number corrected, mean gas transfer velocity is 13.7 ± 4.1 cm h−1. The new approach, based on surface roughness, represents a potential alternative to commonly used parameterizations based on wind speed.